Locking cap assembly for spinal fixation instrumentation

ABSTRACT

A spinal anchor assembly for securing a spinal fixation element has an anchor element adapted for attachment to bone. The anchor element has an opening for receiving a spinal fixation element and side walls on opposed sides of the opening with a plurality of inferior surfaces defined on the side walls. A twist-lock closure closes the opening and captures the spinal fixation element. A locking element can be provided on the closure element and the anchor element. The locking element can be configured so that when the closure element is rotated from the open position to the closed position, the locking element engages to deter the closure element from twisting back to the open position.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/421,921, filed Jun. 2, 2006, and entitled “Locking Cap Assembly forSpinal Fixation Instrumentation”, which is a continuation of U.S. patentapplication Ser. No. 10/828,508, filed on Apr. 20, 2004, and entitled“Locking Cap Assembly for Spinal Fixation Instrumentation”, which is acontinuation of U.S. patent application Ser. No. 09/667,937 to Bono etal., filed on Sep. 22, 2000, and entitled “Lock Cap Anchor Assembly forOrthopaedic Fixation,” which applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to orthopedic fastening systems and tomechanisms for securing and locking a linking or stabilizing element,such as a rod, to a bone screw having a slotted head that receives therod therein. It also relates to structures or anchor assemblies havingsuch a slotted or open head for receiving a rod, wherein the slottedportion extends from a hook, plate, bracket or positioning arm.

A number of such structures are known. Thus, for example, the widelyused Harms T-plate used for stabilizing the cervical vertebrae has aprojecting slotted bolt adapted to receive a rod or cable through theslot and clamp down by screwing a nut along the bolt to bear downagainst the rod. Several patents show holding structures for a fixationrod that are incorporated in the head of a screw, as in U.S. Pat. No.5,672,176, or into a small offset plate which itself may be fastened tothe bone, as shown in published International Application WO96/28105.Other systems involve hooks, transverse rod connectors, or tandemconnectors. Various tools have been provided for these systems to enablethe surgeon to bend and shape the rod to a desired contour in situ, toposition the rod in the slot of a bolt or head, and to secure the rod inposition.

Because the rod is the stabilizing member which provides a precisecontour, spacing or connection between one or more vertebrae, bones orbone fragments, alignment is quite critical, and the ability to pass therod through two or more connecting assemblies requires various actionsto form and shape the rod, or align the receiving structures at definedpositions or path before final clamping is effected. This may involvepositioning and removing the rod several times to check and adjust thedegree of alignment. Thus, it is generally desirable to have a closureor secure locking mechanism that may be effected with simpleinstallation steps.

One generally accepted locking mechanism simply involves an internallythreaded locking nut that may be tightened down along the axis of thescrew or slotted shaft, using a tool such as a socket wrench. Anothercommercial device employs a bayonet-mount cap that captures or iscaptured by the screw head, as in U.S. Pat. Nos. 5,346,493 and5,257,993. Another system utilizes a cap element with a dovetail ordovetail channel that slides over the rod to close the top of the slotand wedge the rod firmly in position. This latter construction involvesno rotation of threaded members, but has the disadvantage that a certainamount of unobstructed lateral space along the rod adjacent to theconnection point is necessary for the sliding installation of theclosure cap. Furthermore, the cap inserts or sliding wedge closures,while they eliminate the need for awkward screwing or rotational motionduring installation, cannot be used with some existing reduction screws,translation hooks or other common hardware having lengthy protrudingguide members, reduction tabs or the like. Moreover, the wedge/capclosures are a specialized component that may require the user to switchentirely over to a proprietary line of orthopaedic hardware if he is toutilize the full range of hook, tab, plate and screw fixation pointsthat may be required in spinal surgery. While the closure systemsdescribed above have in at least some instances been quite successful,it would be beneficial to provide a closure assembly that could securelylock down a rod down while requiring only a small number of lockingsteps by the surgeon and small lateral clearances surrounding theclosure.

SUMMARY OF THE INVENTION

One or more of these and other desirable traits are achieved inaccordance with the present invention by a fixation assembly wherein aclosure cap fits over an opening to close a rod-receiving slot of afixation screw, hook, post or other anchor assembly, and capture the rodtherein. A set screw can be threaded through the cap and tightenedagainst the rod to further clamp it in the assembly. In one embodiment,the closure cap extends over and around the head of the rod-receivingassembly, which may, for example include a slotted shaft, post or head,and the cap is adapted to lock together therewith by limited rotation.This may be accomplished in one embodiment construction by arranging thecap to have a set of sector rim protrusions positioned to fit through acorresponding set of protruding bosses or partial flange segmentslocated on the head, and to rotate into opposition therewith forsecuring the cap onto the top of the fixation screw. The protrusions orflange segments are angled, along the radial direction, so that theybear against each other and jam when rotated, thus cannot slip out ofengagement. The opposed segments tighten and lock the cap against thehead when the cap is rotated through a partial turn of abouttwenty-three degrees of arc, like a flange-locking bayonet mount. Thecap may have a rim that extends over the outside of the bolt head toengage external flange segments on the head. In a preferred embodiment,the mating portions may located internally in the head, with flangesegments projecting radially inward from the perimeter, and the capfitting between segments in the head of the bolt and locking with atwist-in motion to capture a rod in the opening. A hold-down set screwthreads through a central opening in the cap and tightens down againstthe rod to clamp the rod firmly in place. The closing and clamping mayeach be effected by a driver tool that operates along the axis of theassembly and requires little or no side clearance to rotate either thecap or the set screw. In the case of the external, twist□on cap, thelimited degree of rotation allows the cap to also include slots throughthe cap to accommodate reduction tabs extending upwardly from theunderlying screw or anchor member. The quick-twist closure capassemblies of the invention may be adapted to a wide range of screw,hook, eye, plate, connector and other anchor assemblies for rod, cableand other linking elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings:

FIGS. 1A-1C schematically illustrate various spinal fixation rod anchorassemblies of the prior art;

FIG. 2 shows an embodiment of an anchor screw of the present invention;

FIGS. 3 and 3A-3C show views of a closure cap utilized with the anchorscrew of FIG. 2 in accordance with the present invention;

FIG. 4 shows a second embodiment of an anchor member and closure cap ofthe invention;

FIG. 4A is a perspective view from below of the cap of FIG. 4;

FIG. 4B illustrates details of the anchor member of FIG. 4;

FIG. 5 shows a third embodiment of an anchor member and closure cap ofthe invention;

FIGS. 6A, 6B and 6C illustrate the cap and head structure, respectively,of embodiment of FIG. 5 in greater detail; and

FIGS. 7A, 7B and 7C illustrate cap, head and assembled structure,respectively, of another internal twist cap embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention and its range of embodiments will be better understoodfollowing a brief description of prior art, illustrating approaches toone- and two-part anchor assemblies, as well as certain commonconstructions.

FIGS. 1A-1C illustrate prior art anchor mechanisms for securing afixation rod as used in spinal fixation. FIG. 1A illustrates an anchorscrew, while FIG. 1B shows an offset tab having a slotted post forreceiving the rod and FIG. 1C shows a combined system in which anchorscrews secure rod-shaped offset elements that, in turn, each terminatein an end that grips a common fixation rod. In the anchor screw of FIG.1A, a screw 1 with a rounded head 2 carries a slotted top member 3 intowhich a rod 5 shown in phantom is clamped by screwing down a threadedpress ring or set screw 6. The press ring 6 is turned by engagementalong its central portion e.g., by an Allen wrench, and has externalthreads 6 a which fit corresponding internal threads 3 a of the topmember 3. A separate body 7 fills the space between the inner wall ofthe top 3 and the ball head 2 of the screw 1, so that when the rod ispressed down by the member 6, the screw head is firmly gripped and allparts are rigidly held together. An external nut 8 threads over theoutside of the top to further strengthen and lock the assembly. For thisprior art anchor member, the screw 1, the press member 6 and the nut 8may all be installed with a straight tool, such as an Allen wrench orsocket wrench, inserted directly along the axis of the screw.

FIG. 1B shows another anchor assembly 15 for receiving a fixation rod 5.In this assembly, an offset tab construction having a body 15 a that isanchored by a conventional bone screw 10 and including a slotted post(not numbered) for receiving the rod, is closed by a cap nut 17 whichcarries a pressure member 18 centrally thereon to press down against therod 5 as the nut is tightened. In each of these two constructions, themember 6 or 17 for clamping down against the rod 5 installs byrotational movement.

Another prior art anchor assembly is illustrated in FIG. 1C. In thisarticle, a slotted body 21 or 22 is carried either on a bone screw (notvisible in the Figure) or on a short length of offset rod 22 a. In bothcases, the slotted body 21 or 22 receives a rod and clamps it tightly.In this assembly the slotted head member 21 or 22 has angled ordovetailed walls at its upper portion, and a correspondingly shapedsliding cap member 23 is pressed along the dovetail into the upperregion, sliding along the axis of the slot to close the slot and wedgefirmly against the rod passing therethrough. As noted above, thisconstruction has a disadvantage that a lateral clearance along thelength of the rod is necessary for movement of the closure member 23into position. Other constructions are shown in U.S. Pat. Nos.5,346,493, 5,257,993 and elsewhere.

Thus, the art includes both one-piece, and many-piece anchor assemblies,and these may look like screws, or may be specialized elements that arethemselves to be anchored by another assembly. As described furtherbelow, the present invention provides a closing and fixing mechanism ofenhanced utility, with a structure adaptable to much of this broad rangeof hooks, screws, connector assemblies and other orthopaedic anchorhardware involving one or more rod, cable, wire or other linkingelements.

FIG. 2 illustrates a first embodiment of an anchor screw assembly 31 ofthe present invention. As shown, the anchor screw assembly 31 includes ascrew 32 and a top member 33 which may be integral with the screw or,like the prior art construction of FIG. 1A, may be a separate headmember that secures to the proximal end of the screw 32. The top member33 includes a slot indicated generally by 34 for receiving a rod, andcontains at its uppermost region 35, a plurality of segmented or partialflange members 36 a, 36 b, 36 c, 36 d which extend radially outward fromits perimeter and have respective slots or spaces 37 a, 37 btherebetween. As further shown in FIG. 2, each of the flange segments 36a, 36 b, 36 c, 36 d has a lower surface 38, as best seen in the endviews of flange segments 36 a and 36 c, that engages a closure cap 40(FIGS. 3A-3C). While not shown, one or more of the flange segments orcap may include a notch, detent or catch or a jamming feature, toprevent rotation in the opposite sense.

The anchor screw or hook 31 of FIG. 2 is used in conjunction with aclosure cap 40 which is shown in an upward-facing view, from below, inFIG. 3C. The cap 40 fits over and around the upper portion 35 of theslotted, rod receiving top member. As shown FIG. 3, the cap 40 includesa body 41 having a central threaded bore 42 extending therethrough and apair of dependent side members 43 a, 43 b on opposed peripheral sidesthereof which extend downward on opposed sides surrounding the outercircumference of the top member 33. Each of the side members 43 a, 43 bcarries mating inwardly directed protrusions 46 a, 46 b, and 46 c, 46 d,respectively, which are spaced apart and positioned to correspond to thesegmented flange members 36 a-36 d of the screw head. In particular, theprotrusions 46 a to 46 d are positioned below the main body of the cap41 by an amount corresponding to the maximum thickness of the flangesegments 36 a to 36 d, and are rotationally offset so as to pass downthrough the gaps between segments and rotate into gripping engagementaround the segments by a partial rotation of the cap 40 about the screwhead assembly or top member 33, in the manner of a bayonet mounted lidclosure. This secures the cap 40 on the top member closing the slot toprevent movement of the rod or cable from the head along the axialdirection of the screw 32. A set screw (not shown) threaded through theaperture 42 is then tightened to clamp firmly down against the rod,cable or other linkage captured in the slot 34 (FIG. 2).

Advantageously, with the foregoing construction, the anchor screw 31 andthe closure assembly 40 as well as the set screw (not illustrated) allinstall by simple rotational movement of a tool that extends directlyalong the axis of the screw. Moreover, as illustrated, the initiallocking of the cap on the head assembly is effected by a smallrotational movement, substantially less than one-half turn, whichcorresponds approximately to the length of the lower surface 38 of oneflange segment, or about 20 degrees of rotational movement. Thus a veryslight movement is sufficient to capture the rod 5 (FIGS. 1A-1C) withinthe slot 34 during initial setup or fitting of the fixation rod.

This twist-lock flanged anchoring assembly with a cap structure of thepresent invention is readily adapted to diverse other fixation screws ofknown design, and thus in various alternative embodiments andadaptations may carry forward the advantages of those other designs.Thus, for example, the locking cap assembly of FIGS. 2-3 of theinvention may be adapted to an anchor assembly such as a reductionscrew, anchor screw, or hook in which the anchor member possessesprotruding reduction tabs that extend upwardly from the head of theanchoring assembly.

Such an embodiment 50 is shown in FIG. 4. In this embodiment, the head53 of the anchor assembly has a pair of reduction tabs 55 extendingupwardly from the sides of the slot. In this case, the inventioncontemplates a closure cap 60 with a rim-engaging securing structuresimilar to that of cap 40 for engagement by a small rotational motion,but the cap structure further includes a pair of arcuate slots 62 a, 62b located in its central region and sized for passage of the reductiontabs 55 or other protruding head structure therethrough. Each of theslots 62 a, 62 b extends past the edges of the tabs 55, permittingsufficient rotation of the cap to lock the cap in position. Thestructure of the cap itself strengthens or supports both the surroundingwall of the rod receiving slot, and the thin-walled tabs 55 which risetherefrom, while leaving the central on-axis region above the capentirely unobstructed for insertion, for example, of a set screw alongan axial direction, and permitting line-of-sight access by a driver forinstallation.

In any of the foregoing constructions, the rod-receiving head assemblyor top member 33, 53 may be integral with the anchor screw 35, 52 or maybe constituted by a separate slotted head member that fits about the topof the screw to grip the rod or other connecting linkage. Thus, theinvention applies to diverse anchors, hooks, monoaxial screws,transverse connections or tandem connections, slotted connectors or thelike.

FIG. 4 illustrates this aspect of the invention for a reduction tabembodiment of which the head is separate from the screw. As shown, theanchor screw assembly 50 has a screw body 52 with an enlarged head 54which may, for example, have an Allen or other female socket formedtherein (not shown) for applying torque along the axis of the screw toinsert the screw in bone. A slotted top member 53 having a taperedinterior bore is first fitted over the head 54, receiving the screw bodyfrom above, capturing the enlarged ball head 54 of the screw therein. Acompressed member (not shown) which may be similar to element 7 of FIG.1A, may be provided to create a binding fit, and this element may befixed in place, for example, by swaging at opposed surface reliefdrillings 53 a, or it may fit by simple compression. The provision oftop member 53 as a separate head structure that is loosely fixed to, butoriginally decoupled from, the screw 52 in this manner allows the slotangle to be set at a later stage of installation, while avoiding therisk of losing separate small components.

The cap 60 of this embodiment, which is shown in a perspective view frombelow in FIG. 4A, is similar to that of the first described embodiment,but includes arcuate slots 62 a, 62 b to accommodate the projectingreduction tabs. In each case, the cap member having a dependent lockingrim that grips the outside of the slotted top and closes the slot by apartial rotation, provides a simple and unobstructed procedure forclosing the head of the anchor and capturing the rod, cable or otherlinkage in the anchor assembly and clamping the linkage while fixing theorientation.

In each of the foregoing illustrated embodiments, the cap extendsradially beyond the outer radius of the anchor screw head assembly, andhas a rim that extends to a greater diameter, and slides between thesegmented flange bosses 36 to rotate into a captured position whichcloses the slot and captures the rod or other linkage within the head ofthe anchor assembly. A radial slant “RS” at an angle Θ may be providedon one or more faces of the opposed locking members as shown in thedetail FIG. 4B to assure that they cannot slip radially outward underpressure.

In further embodiments, the invention contemplates a twist-on cap memberwhich fits within the head of the anchor assembly rather than extendingover and locking on the outside of the head.

FIG. 5 illustrates one embodiment 100 of such a twist-in anchor closingmechanism. As shown, the anchor assembly 100 has a screw portion 152 foranchoring in bone, and a head portion 153 for receiving the rod, cableor other linkage. A closure cap 140 closes the slotted end of head 153.As in the previously described embodiments, the screw and head may beseparate assemblies, in which case the upper portion of the screwpreferably has a ball end as described above that allows the head topivot about the axis of the screw and achieve a further degree offreedom in angular orientation before clamping down. As with the earlierdescribed embodiments, the cap or closure portion 140 may have a centralbore 145 which is internally threaded to accommodate a set screw tofurther clamp the rod in the slot; however, to simplify the drawing,threads are omitted from FIG. 5.

The internal closure cap 140 has a plurality of radially protrudingflange segments 146, of which one is visible in the Figure, and the capis pushed downwardly on the head so the respective inward and outwarddirected segments pass between each other, in a manner similar to theabove-described embodiments. Thus, the segments 146 fit betweencorresponding inwardly protruding segments 158 of the head 153 and lockthereagainst by a small rotation of the cap 140.

FIGS. 6A and 6B illustrate the structure of the twist-in cap 140 and theslotted head 153 in greater detail.

FIG. 6A shows the closure cap 140, and FIG. 6B shows the head assembly153, of an internal closure locking cap of FIG. 5. As shown, the headassembly 153 of the anchor screw has a plurality of internallyprojecting bosses 158 and the closure cap 140 has correspondingoutwardly projecting bosses 146. Respective bosses 146, 158 aredimensioned such that the cap 140 may be pushed downwardly betweenspaces of corresponding bosses to position the upper surface 149 of thecap bosses below the lower surface 159 of the retaining head bosses 158.As shown in FIGS. 6A and 6B, these mating contact surfaces are angled orsloped downwardly with a radial slant “RS” at an angle Θ (illustrated inFIGS. 5 and 6B) toward the center. In this way, when the cap 140 isrotated to place opposed bosses in an engagement with each other, thecap exerts a net inwardly directed force on the head to preventspreading of the retaining slot. The contours of the sloped ends arerelatively sharply defined, effectively forming a circumferential ridge149 a, 159 a and groove 149 b, 159 b on each of the respectivecomponents (FIG. 6C). The ridge of one part fits in the groove of theother, so that the closure is centered and grips over a substantialcontact area.

As best seen in FIGS. 5 and 6A, the twist-in cap has opposed edge flats144 which may provide a contact or engagement surface for a tool such asa wrench used for turning the cap upon installation. Each of the flats144 has a corner to prevent over-rotation of the cap, so that uponinsertion it rotates to exactly position the respective bosses 146, 158opposite each other as the anchor assembly is closed. The set screw isthen tightened to secure the fixation linkage captured in the slot.

FIGS. 7A-7C illustrate another embodiment of a twist-in closure cap andspinal anchor assembly, having an anchor screw, a slotted head and atwist-in closure cap. As in the previously described embodiments, thescrew and head may be separate assemblies, e.g., to achieve freedom inangular orientation before clamping down. The head and cap structuresimilarly may be adapted to spinal anchor assemblies of other types,such as transverse connectors, anchor plates and other link-receivinghardware. As with the earlier described embodiments, the cap may have acentral bore which is internally threaded to accommodate a clamping setscrew to lock and immobilize the linkage once it has been captured inthe slot; however, to simplify the drawing, threads are omitted fromFIGS. 7A and 7C.

In this embodiment, the radially protruding bosses or flange segments146 of the cap, and the inwardly protruding bosses 158 of the head arearranged so the respective inward and outward directed segments passbetween each other, when the cap is pushed downwardly into the head, ina manner similar to the above-described embodiments. Thus, the segments146 fit between corresponding, inwardly protruding segments 158 of thehead and lock thereagainst by a small rotation of the cap. In addition,the twist-lock mechanism may be configured to exert enhanced contactforce in a detent region when the clamp screw is tightened down.

As shown in FIGS. 7A-C, this is achieved in a presently preferredembodiment by providing lower and upper contact faces 159, 149 on thesegments 158, 146 of the head and cap, respectively, that slope downwardtoward the center so that when the set screw is tightened the upwardforce on the cap draws the segments 158 inward and upward. Avertically-oriented protruding ridge 158 a and mating groove 146 a areformed on the head and cap, on or directly above the correspondingflange region of each, so that the ridge 158 a on the head is urgedinwardly against the groove surface of the cap. This effectively locksthe rotational detent to prevent any rotational movement of the cap oncethe set screw is tightened.

As further seen in FIGS. 7A and 7C, the twist-in cap has a protrudingstop face 164 that contacts the head and prevents over-rotation of thecap when it is turned to close the head. Thus, upon insertion the caprotates (clockwise as shown) to position the bosses 146, 158 exactlyopposite each other as the anchor assembly is closed. The set screw isthen tightened to secure the fixation linkage captured in the slot.

The invention being thus disclosed and illustrative embodiments depictedherein, further variations and modifications of the invention will occurto those skilled in the art. All such variations and modifications areconsidered to be within the scope of the invention, as defined by theclaims appended hereto and equivalents thereof.

What is claimed is:
 1. An anchor assembly for securing a linkage,wherein the anchor assembly comprises: an anchor element configured forattachment to a bone, and having a head having an open slot forreceiving the linkage; a cap for closing the open slot; a first lockingelement disposed on the cap; and a second locking element disposed onthe anchor element; wherein the first locking element and the secondlocking element are configured to twist-lock together by a partialrotation of the cap within the anchor element to cover the slot so as tocapture the linkage.
 2. The assembly of claim 1, wherein the cap furtherincludes a central clamping member for tightening down to secure thelinkage.
 3. The assembly of claim 2, wherein the central clampingelement is independently operable with respect to the cap.
 4. Theassembly of claim 3, wherein the assembly is configured so that the capcan be placed in a closed position to movably capture a linkage withinthe head and the central clamping element can be independently operatedto fix the position of the linkage within the head.
 5. The assembly ofclaim 4, wherein the central clamping element is rotatably connected tothe cap.
 6. The assembly of claim 5, wherein the central clamping memberis a central clamping bolt.
 7. The assembly of claim 1, wherein thefirst and second locking elements are configured to lock upon rotationof the cap from an open position to a closed position.
 8. The assemblyof claim 7, wherein, upon locking, the first and second locking elementsdeter the cap from rotating back to the open position.
 9. The assemblyof claim 1, wherein the head includes at least two side walls definingat least a portion of the open slot, inferior contact surfaces aredefined in the side walls, and the cap includes flange segments thatinteract with the inferior surfaces in a closed position to close theopen slot.
 10. The assembly of claim 9, wherein the inferior contactsurfaces extend in a direction that defines a radial slant and the capflanges include a superior contact surface extending at an anglecorresponding to the radial slant when the closure element is placed inthe closed position.
 11. The assembly of claim 10, wherein the radialslant is configured so that when the cap flange superior surfaces engagethe inferior surfaces when the closure element is placed in the closedposition in the anchor element open slot and pressure is applied tosecure the linkage within the open slot in the anchor element, theanchor element side walls and cap flanges are drawn together.
 12. Theassembly of claim 9, wherein a plurality of inferior surfaces areprovided on each side wall.
 13. The assembly of claim 12, wherein theplurality of inferior surfaces provided on each side wall are configuredso that the closure element can be moved from the open to the closedposition by twisting the closure element by less than or equal to abouttwenty degrees.